I quite often see singletons in code, and I think they are overused.

Singletons have the disadvantage that everyone in the whole world has
access to that object, which is as bad as a global function. Anyone can
call a singleton and create havoc on it.

Singletons also introduce hard-coded dependencies that are difficult to
break if one wishes to change the environment or reuse some of the
classes in a different project.

Just because you call something a "singleton" instead of a global
variable does not make it a good pattern.

Singletons are lazy design, since "global scope" is too broad a scope
for most objects.

So my point is this: singletons should be discouraged, even banned.

Calum

From:	H. S. Lahman
Subject:	Re: Singletons
Date:	Sat, 15 Jan 2005 21:29:46 GMT

Responding to Grant...

> I quite often see singletons in code, and I think they are overused.

That is very true. They should only be used when two conditions both
prevail:

(1) The must never be more than one instance of the object.

(2) There are multiple opportunities to create an instance during
execution because

(2a) An instance can be created in different contexts (i.e., by
different objects) and/or

(2b) The instantiating behavior can be invoked multiple times.

However, when those conditions prevail Singleton is usually the best
(safest and most readily understood) mechanism.

>
> Singletons have the disadvantage that everyone in the whole world has
> access to that object, which is as bad as a global function. Anyone can
> call a singleton and create havoc on it.

All public object attributes are global in this sense. They can be
accessed by any object in the application having a valid relationship
navigation path to traverse to reach the target object. But...

>
> Singletons also introduce hard-coded dependencies that are difficult to
> break if one wishes to change the environment or reuse some of the
> classes in a different project.
>
> Just because you call something a "singleton" instead of a global
> variable does not make it a good pattern.
>
> Singletons are lazy design, since "global scope" is too broad a scope
> for most objects.

This ignores that the primary technique for managing persistent state in
OO development is relationship instantiation. One can only reach
objects to access their state variables if a relationship path exists to
reach them.

1 paid by R1 *
[Department] ------------------- [Professor]
| 1 | 1
| majors in | taught by
| |
| R2 | R3
| |
| * R4 | *
[Student] ---------------------- [Course]
* | attends *
|
[Registration]

Now when a Professor seeks to grade a Student, the Professor is limited
to those Students that can be accessed by the R3 -> R4 relationship
navigation, which will be a smallish subset of all Students. It is up
to the developer to ensure that the dynamics navigate the correct
relationship path during collaborations. Thus grading all Students
reached by R1 -> R2 would be invalid because the resulting set of
Students via that path is a superset of the Students resulting from the
R3 -> R4 path that restricts grading to Course that the Professor teaches.

*************
There is nothing wrong with me that could
not be cured by a capful of Drano.

H. S. Lahman
hsl@pathfindermda.com
Pathfinder Solutions -- Put MDA to Work
http://www.pathfindermda.com
blog (under constr): http://pathfinderpeople.blogs.com/hslahman
(888)-OOA-PATH

From:	Jimmy Cerra
Subject:	Re: Singletons
Date:	Sun, 16 Jan 2005 01:08:33 GMT

H. S. Lahman wrote:
> That is very true. They should only be used when two conditions both
> prevail:
>
> (1) The must never be more than one instance of the object.
>
> (2) There are multiple opportunities to create an instance during
> execution because
>
> (2a) An instance can be created in different contexts (i.e., by
> different objects) and/or
>
> (2b) The instantiating behavior can be invoked multiple times.
>
> However, when those conditions prevail Singleton is usually the best
> (safest and most readily understood) mechanism.

Do objects that manage the creation of other objects usually make good
Singletons? Like Abstract Factories implementations, Object
Pools/Managers, and the like?

In fact, I think this lets the singleton be more flexible and less
risky. That is, if you think a class should be a singleton, make the
constructor protected and allow it to be managed by a seperate singleton
object manager. If you change your mind, you only have to change the
manager's code, or even just write another manager. E.G.

Say you origionally think Window objects hould be singletons. So write
this instead of the classic pattern. (note: there are better Singleton
implementations... I chose this one for brevity):

] package Motif;
] public class Window {
] protected Window() {...}
] }

] package Motif;
] import Motif.Window;
] public class WindowManager {
] private static WindowManager wm = new WindowManager();
] private static Window ww = new Window();
] protected WindowManager() {...}
] public static getInstance() {return wm;}
] public getWindow() {return ww;}
] }

If you change your mind, then either change WindowManager or (better)
create a new manager:

] package Motif;
] import Motif.Window;
] public class WindowManager2 {
] private static WindowManager2 wm = new WindowManager2();
] protected WindowManager2() {...}
] public static getInstance() {return wm;}
] public getWindow() {return new Window();}
] }

It also allows Window objects to be easily extended (singletons are
traditionally hard to extend or subclass).

--
Jimmy Cerra
https://nemo.dev.java.net/

From:	H. S. Lahman
Subject:	Re: Singletons
Date:	Tue, 18 Jan 2005 04:25:55 GMT

Responding to Cerra...

>> That is very true. They should only be used when two conditions both
>> prevail:
>>
>> (1) The must never be more than one instance of the object.
>>
>> (2) There are multiple opportunities to create an instance during
>> execution because
>>
>> (2a) An instance can be created in different contexts (i.e., by
>> different objects) and/or
>>
>> (2b) The instantiating behavior can be invoked multiple times.
>>
>> However, when those conditions prevail Singleton is usually the best
>> (safest and most readily understood) mechanism.
>
>
> Do objects that manage the creation of other objects usually make good
> Singletons? Like Abstract Factories implementations, Object
> Pools/Managers, and the like?

Usually not. That's because concrete factories are usually instantiated
once during application initialization. There is usually no need for
more than one and it will be more efficient to allocate it once at
startup than to allocate/deallocate it on an as-needed basis.

OTOH, I can envision situations where it might be desirable to create
factories on an as-needed basis and get rid of them afterwards. In that
case one might have a need for making them Singletons for efficiency
sake because the as-needed invocation may be unpredictable or repeated.

>
> In fact, I think this lets the singleton be more flexible and less
> risky. That is, if you think a class should be a singleton, make the
> constructor protected and allow it to be managed by a seperate singleton
> object manager. If you change your mind, you only have to change the
> manager's code, or even just write another manager. E.G.

Hmmm. OK, I think I may have answered a different question above than
you asked. B-)

If you just mean that if one is already using a factory to create
instances, then let the factory control the number rather than a static
method in the class being instantiated, then I agree. That is
necessarily more flexible because the factory has more control over the
instance being created (i.e., in Singleton there is only one possible
flavor of instance to create initially)

However, I would counter by wondering how much need there is for that
flexibility. The only situation I can think of where that would be
useful is when one needs one sort of single instance (read: attribute
initialization) for awhile during some context but when that context is
replaced one needs a different sort of single instance for awhile. IOW,
one wants to be able to delete the current flavor and recreate a new flavor.

But one can do that if one adds a static delete method to Singleton.
The odds are somebody else needs to understand when the context switch
and whoever that is can invoke the delete() method of the singleton.
Taking that one step further, whoever that is also likely to understand
what new flavor of single instance is needed for the new context. So
they would be a single point of instantiation and one wouldn't need a
Singleton at all. B-)

>
> Say you origionally think Window objects hould be singletons. So write
> this instead of the classic pattern. (note: there are better Singleton
> implementations... I chose this one for brevity):
>
> ] package Motif;
> ] public class Window {
> ] protected Window() {...}
> ] }
>
> ] package Motif;
> ] import Motif.Window;
> ] public class WindowManager {
> ] private static WindowManager wm = new WindowManager();
> ] private static Window ww = new Window();
> ] protected WindowManager() {...}
> ] public static getInstance() {return wm;}
> ] public getWindow() {return ww;}
> ] }

OK. But unless I'm missing something this is just a Singleton
(WindowManager) combined with a composition relationship

1
[WindowManager] <*>--------------- [Window]

The Whole/Part composition with unconditional 1:1 multiplicity already
ensures there can't be more than one Window created for each
WindowManager, so if WindowManager is a Singleton there is only one
point of instantiation and Window does not need to be a Singleton.

>
> If you change your mind, then either change WindowManager or (better)
> create a new manager:
>
> ] package Motif;
> ] import Motif.Window;
> ] public class WindowManager2 {
> ] private static WindowManager2 wm = new WindowManager2();
> ] protected WindowManager2() {...}
> ] public static getInstance() {return wm;}
> ] public getWindow() {return new Window();}
> ] }
>

OK, but WindowManager2 is a different class than WindowManager. I'm not
sure I see how this is different than having any two different classes
that are singletons.

> It also allows Window objects to be easily extended (singletons are
> traditionally hard to extend or subclass).

I don't think subclassing should be a problem; only a leaf subclass is
instantiated, so any one or all of the leaf subclasses can be a
Singleton. That is, subclassing, inheritance, and polymorphism are all
orthogonal to the number of instances of the leaf subclasses.

*************
There is nothing wrong with me that could
not be cured by a capful of Drano.

H. S. Lahman
hsl@pathfindermda.com
Pathfinder Solutions -- Put MDA to Work
http://www.pathfindermda.com
blog (under constr): http://pathfinderpeople.blogs.com/hslahman
(888)-OOA-PATH

From:	Jimmy Cerra
Subject:	Re: Singletons
Date:	Tue, 18 Jan 2005 05:34:14 GMT

H. S. Lahman wrote:
>> Do objects that manage the creation of other objects usually make
>> good Singletons? Like Abstract Factories implementations,
>> Object Pools/Managers, and the like?
>
> Usually not.

Why? GoF explictly states that factories make good Singletons. Mark
Grand in "Patterns in Java Volume 1" says the same for Object Pools.
Are they incorrect? I'm not as knowledgable as them (or you) so I don't
know for certain, but that's what I read.

>> In fact, I think this lets the singleton be more flexible and less
>> risky. That is, if you think a class should be a singleton, make the
>> constructor protected and allow it to be managed by a seperate
>> singleton object manager. If you change your mind, you only have to
>> change the manager's code, or even just write another manager. E.G.
>
> If you just mean that if one is already using a factory to create
> instances, then let the factory control the number rather than a
> static method in the class being instantiated, then I agree. That is
> necessarily more flexible because the factory has more control over
> the instance being created (i.e., in Singleton there is only one
> possible flavor of instance to create initially)

That's basically it.

> However, I would counter by wondering how much need there is for that
> flexibility. The only situation I can think of where that would be
> useful is when one needs one sort of single instance (read: attribute
> initialization) for awhile during some context but when that context
> is replaced one needs a different sort of single instance for awhile.
> IOW, one wants to be able to delete the current flavor and recreate a
> new flavor.

I was thinking of the situations when once an object should have been a
singleton. Then later on the singleton behavior needed to be factored
out, because the origional programmers were mistaken during the previous
design. However, it is hard to "unsingleton" classes when some code
still depends on the singleton behavior.

Therefore, I'm thinking that the singleton behavior should be placed
into an external object like a factory. Then to remove the singleton
behavior, one just needs to write a new factory. Both applications the
old and new code can reuse the same source without modification. I
think this is preferable to changing existing code.

> But one can do that if one adds a static delete method to Singleton.
> The odds are somebody else needs to understand when the context switch
> and whoever that is can invoke the delete() method of the singleton.
> Taking that one step further, whoever that is also likely to
> understand what new flavor of single instance is needed for the new
> context. So they would be a single point of instantiation and one
> wouldn't need a Singleton at all. B-)

Can you reexplain that?

> OK. But unless I'm missing something this is just a Singleton
> (WindowManager) combined with a composition relationship
>
> 1
> [WindowManager] <*>--------------- [Window]

The idea is that we are certain that WindowManager can be a singleton
(factories often make good singletons, according to GoF), while we are
less certain that only one Window will only ever need to be created.

> OK, but WindowManager2 is a different class than WindowManager. I'm
> not sure I see how this is different than having any two different
> classes that are singletons.

Say Window was a singleton:

] package Motif;
] public class Window {
] private static Window ww = new Window();
] public static getInstance() {return ww;}
] private Window() {...}
] }

Then several framework revisions later, we find this to be an error.
Old code still depends on the singleton behavior, while new code would
be better served with many Windows. It is messy to keep the singleton
behavior and allow the constructor to be private. Factoring it out
later breaks compatibility with old programs.

A more flexible design factors out the singleton behavior while
initially designing Window. Then you can change the behavior by adding
code rather than changing existing classes.

That's the intent of the idiom. I was just thinking outloud, anyway.

> I don't think subclassing should be a problem; only a leaf subclass is
> instantiated, so any one or all of the leaf subclasses can be a
> Singleton. That is, subclassing, inheritance, and polymorphism are
> all orthogonal to the number of instances of the leaf subclasses.

Considerable discussion is devoted to subclassing Singletons in GoF.
Mark Grand in "Patterns in Java Vol. 1" states that extending singletons
is awkward.

--
Jimmy Cerra

From:	H. S. Lahman
Subject:	Re: Singletons
Date:	Tue, 18 Jan 2005 20:30:13 GMT

Responding to Cerra...

> H. S. Lahman wrote:
> >> Do objects that manage the creation of other objects usually make
> >> good Singletons? Like Abstract Factories implementations,
> >> Object Pools/Managers, and the like?
> >
> > Usually not.
>
> Why? GoF explictly states that factories make good Singletons. Mark
> Grand in "Patterns in Java Volume 1" says the same for Object Pools. Are
> they incorrect? I'm not as knowledgable as them (or you) so I don't
> know for certain, but that's what I read.

Making a good Singleton _when necessary_ and determining whether they
need to be Singletons are two quite different things. There usually
aren't multiple opportunities for instantiating a concrete factory, so
there is no need for the complexity. OTOH, when there are multiple
instantiation opportunities, then factories are good candidates for
Singleton.

> I was thinking of the situations when once an object should have been a
> singleton. Then later on the singleton behavior needed to be factored
> out, because the origional programmers were mistaken during the previous
> design. However, it is hard to "unsingleton" classes when some code
> still depends on the singleton behavior.
>
> Therefore, I'm thinking that the singleton behavior should be placed
> into an external object like a factory. Then to remove the singleton
> behavior, one just needs to write a new factory. Both applications the
> old and new code can reuse the same source without modification. I
> think this is preferable to changing existing code.

But who is accessing the instance() behavior? That will only be those
who are creating it or instantiating relationships with it when
instantiating other objects (i.e., factories). If there is a change in
the requirements (or the original design was in error), they are going
to have to be modified anyway.

Just in case, let me belabor the difference between instantiation and
navigation of relationships. The instance() method should never be
invoked simply to navigate a relationship during collaboration. When an
instance is created, all of the necessary relationships with that
instance should be instantiated as well. For example, reference
attributes that implement the relationships will be assigned the address
returned by the instance() function when the relationships are instantiated.

Then any collaborations with the instance by other objects during the
solution execution should be done by navigating the existing
relationships to get to it. So we have:

* 1 1 *
[X] ------------- [Y] ---------------- [Z]

If Y must be a Singleton that is completely independent of the
relationships with [X] and [Z] that are navigated during collaboration.
If Y happens to be a single instance, then every [X] and [Z] instance
will just happen to have a relationship to exactly the same instance of
[Y]. When an X or Z needs to collaborate with its Y it will navigate
the relationship that was instantiated with it or the [Y] instance was
instantiated. This is no different than any other relationship
navigation between collaborating objects; the Singleton just ensures all
relationships end up with one instance reference when they are instantiated.

[Note that when a [X] or [Y] instance is created after a Y exists, the
relationship must be instantiated. In that case it may be convenient
for the factory creating the X or Z to invoke the instance() method.
But that is not necessary because the factory itself may have a
relationship with the Y that is instantiated by the [Y] factory.]

>
> > But one can do that if one adds a static delete method to Singleton.
> > The odds are somebody else needs to understand when the context switch
> > and whoever that is can invoke the delete() method of the singleton.
> > Taking that one step further, whoever that is also likely to
> > understand what new flavor of single instance is needed for the new
> > context. So they would be a single point of instantiation and one
> > wouldn't need a Singleton at all. B-)
>
> Can you reexplain that?

Context A: All collaborations with [Y] above need a single Y instance
that is initialized with particular set of attribute values.

Context B: All collaborations with [Y] above need a single Y instance
that is initialized with particular but different set of attribute values.

Context A and B prevail for extended periods of time during the
application execution but they never coexist and they exist serially.
Suppose context A prevails first and one creates a Singleton with the
appropriate values. (Do that parametrically via an argument to
instance(CONTEXT_A).)

Now context B replaces context A. One solution is to delete the [Y]
instance and recreate it by invoking instance(CONTEXT_B). But one needs
a static delete() method in the Singleton that is a no-op if there is no
instance.

[Alternatively, one can provide a context state variable somewhere that
the Singleton instance() method looks at to determine CONTEXT_A vs.
CONTEXT_B. Then if there is no instance, it creates one with the
correct initialization. If there is an instance consistent with the
context, the method does nothing. If there is an instance but it is
inconsistent with the current context, then the current instance is
deleted and an appropriate one is created.]

>
> > OK. But unless I'm missing something this is just a Singleton
> > (WindowManager) combined with a composition relationship
> >
> > 1
> > [WindowManager] <*>--------------- [Window]
>
> The idea is that we are certain that WindowManager can be a singleton
> (factories often make good singletons, according to GoF), while we are
> less certain that only one Window will only ever need to be created.

OK, but the requirements have to be clear on this. Either you need a
single instance or not. That governs the multiplicity of the
relationship, which determines the how many Windows one needs. The
Whole/Part of the composition relationship still ensures that, since
there is only one WindowManager, the number of Windows (ww) that are
embedded in its definition are the only windows around. If the
multiplicity is 1, then there is no opportunity to create more Windows,
so one doesn't need a Singleton for Window even if the requirements say
there must be only a single instance.

>
> > OK, but WindowManager2 is a different class than WindowManager. I'm
> > not sure I see how this is different than having any two different
> > classes that are singletons.
>
> Say Window was a singleton:
>
> ] package Motif;
> ] public class Window {
> ] private static Window ww = new Window();
> ] public static getInstance() {return ww;}
> ] private Window() {...}
> ] }
>
> Then several framework revisions later, we find this to be an error. Old
> code still depends on the singleton behavior, while new code would be
> better served with many Windows. It is messy to keep the singleton
> behavior and allow the constructor to be private. Factoring it out
> later breaks compatibility with old programs.

I push back with my opening point about relationship instantiation vs.
navigation. The relationships must be instantiated correctly and that
is always part of the responsibility for instance creation.
Once the relationships are instantiated properly, the navigation during
collaborations will /always/ get to the right place, regardless of
whether it is a Singleton or not.

Encapsulating the rules and policies of instantiation is one of the
reasons we have factories in the first place. Collaborations should not
need to understand those rules an policies; they should only be
concerned with the nature of the collaboration (e.g., message sent)
rather than whether they are collaborating with the right instance. Nor
should collaborators need to understand the implementation mechanisms of
instantiation (i.e., the instance() method).

So the only objects that need to understand instance() are the objects
that create the instances (e.g., WindowManager) and their relationships.
Those are exactly the same objects that will have to be modified if
one changes the rules and policies of instantiation (e.g., ensuring
Window is has a single instance).

> > I don't think subclassing should be a problem; only a leaf subclass is
> > instantiated, so any one or all of the leaf subclasses can be a
> > Singleton. That is, subclassing, inheritance, and polymorphism are
> > all orthogonal to the number of instances of the leaf subclasses.
>
> Considerable discussion is devoted to subclassing Singletons in GoF.

The problem GoF has is that it is trying support poor OOA/D techniques.
(And possibly catering to poorly formed OOPLs like C++ that allow
superclass instantiation without specifying a specific leaf subclass.)

When an instance is instantiated from a subclassing tree there is no
polymorphic access; one must instantiate a specific leaf subclass
object. If one does that, then there is no problem in inheriting the
instance() method while providing a leaf implementation and having a
local, properly typed _instance pointer for the leaf subclass that the
local implementation of instance() accesses and having it return the
relevant superclass type.

However, that's kind of pointless because the subclasses are superfluous
since the clients can then only access the superclass properties.

The alternative -- which is a very bad OOA/D practice -- is for the
clients to query the instance to determine what subclass it actually
belongs to so that specializations can be accessed. So the bottom line
is: Don't Do That. Define instance() and _instance only at the leaf
subclass level where static class properties should be.

They also seem to assume that one will access instance() during
collaborations -- another poor OOA/D technique, as indicated above.
Instance() exists to instantiate objects, not to provide collaboration
message addressing.

> Mark Grand in "Patterns in Java Vol. 1" states that extending singletons
> is awkward.

Sorry, I don't do Java so I haven't seen it.

*************
There is nothing wrong with me that could
not be cured by a capful of Drano.

H. S. Lahman
hsl@pathfindermda.com
Pathfinder Solutions -- Put MDA to Work
http://www.pathfindermda.com
blog (under constr): http://pathfinderpeople.blogs.com/hslahman
(888)-OOA-PATH

From:	Daniel T.
Subject:	Re: Singletons
Date:	Sat, 15 Jan 2005 15:49:03 GMT

In article ,
Calum Grant wrote:

> I quite often see singletons in code, and I think they are overused.
>
> Singletons have the disadvantage that everyone in the whole world has
> access to that object, which is as bad as a global function. Anyone can
> call a singleton and create havoc on it.
>
> Singletons also introduce hard-coded dependencies that are difficult to
> break if one wishes to change the environment or reuse some of the
> classes in a different project.
>
> Just because you call something a "singleton" instead of a global
> variable does not make it a good pattern.
>
> Singletons are lazy design, since "global scope" is too broad a scope
> for most objects.
>
> So my point is this: singletons should be discouraged, even banned.
>
> Calum

What about a singleton at package scope (as opposed to global scope?)

From:	Ilja Preuß
Subject:	Re: Singletons
Date:	Sat, 15 Jan 2005 16:51:39 +0100

http://butunclebob.com/ArticleS.UncleBob.SingletonVsJustCreateOne

Cheers, Ilja

From:	Thomas G. Marshall
Subject:	Re: Singletons
Date:	Wed, 19 Jan 2005 01:51:12 GMT

Ilja Preuß coughed up:
> http://butunclebob.com/ArticleS.UncleBob.SingletonVsJustCreateOne
>
> Cheers, Ilja

eh. Whole lot of hoopla about nuthin. Singletons shouldn't be overused,
but so what? They aren't the end of the world either. (unlike operator
overloading, which /is/ one of the four horsemen OTA :) )

He mentions his SIS uncle bobism:

public class Singleton
{
private Singleton() {}
private static Singleton instance = new Singleton();
public static Singleton instance() {return instance;}
}

I use this form, but he pointed out to me once that the following is better
(and I agree), but he doesn't use this in his article:

public class Singleton
{
private Singleton() {}
public static final Singleton instance = new Singleton();
}

I really don't see JCO as a particular win at all.

Now this is to the side of whether or not you /should/ use singletons, or
even unenforced lonely instances. That is, if you do get to the point of
Just Create One, then heck, you've made the lonely decision and committed
the sin, you might as well make it un-@#$%up-able by some later engineer and
enforce it.

--
Forgetthesong,I'dratherhavethefrontallobotomy...

From:	Ilja Preuß
Subject:	Re: Singletons
Date:	Wed, 19 Jan 2005 19:28:27 +0100

Thomas G. Marshall wrote:
> That is, if you do
> get to the point of Just Create One, then heck, you've made the
> lonely decision and committed the sin, you might as well make it
> un-@#$%up-able by some later engineer and enforce it.

Sorry, I don't understand what you are getting at here. Are you arguing that
there is no difference in coupling between using a Singletong and a Just
Create One?

Curious, Ilja

From:	Thomas G. Marshall
Subject:	Re: Singletons
Date:	Wed, 19 Jan 2005 18:33:12 GMT

Ilja Preuß coughed up:
> Thomas G. Marshall wrote:
>> That is, if you do
>> get to the point of Just Create One, then heck, you've made the
>> lonely decision and committed the sin, you might as well make it
>> un-@#$%up-able by some later engineer and enforce it.
>
> Sorry, I don't understand what you are getting at here. Are you
> arguing that there is no difference in coupling between using a
> Singletong and a Just Create One?
>
> Curious, Ilja

Coupling, shmupling. No, I'm saying that there is little difference in
building an entire design around a JCO by convention vs. building an entire
design around a JCO by fiat.

Like I said before: "eh."

--
"It's easier to be terrified by an enemy you admire."
-Thufir Hawat, Mentat and Master of Assassins to House Atreides

From:	Ilja Preuß
Subject:	Re: Singletons
Date:	Thu, 20 Jan 2005 13:59:40 +0100

Thomas G. Marshall wrote:
> Ilja Preuß coughed up:
>> Thomas G. Marshall wrote:
>>> That is, if you do
>>> get to the point of Just Create One, then heck, you've made the
>>> lonely decision and committed the sin, you might as well make it
>>> un-@#$%up-able by some later engineer and enforce it.
>>
>> Sorry, I don't understand what you are getting at here. Are you
>> arguing that there is no difference in coupling between using a
>> Singletong and a Just Create One?
>>
>> Curious, Ilja
>
>
> Coupling, shmupling. No, I'm saying that there is little difference
> in building an entire design around a JCO by convention vs. building
> an entire design around a JCO by fiat.

Is "JCO by fiat" the Singleton pattern here? If not, you managed to confuse
me even more... ;)

BTW, I think that the whole point of JCO is that it's exactly *not* the
entire design that is build around it. Actually almost all of its clients
don't know about the number of instances.

> Like I said before: "eh."

Yes, you keep saying. Doesn't seem to help me understand your point,
though...

Cheers, Ilja

From:	Thomas G. Marshall
Subject:	Re: Singletons
Date:	Thu, 20 Jan 2005 17:05:59 GMT

Ilja Preuß coughed up:
> Thomas G. Marshall wrote:
>> Ilja Preuß coughed up:
>>> Thomas G. Marshall wrote:
>>>> That is, if you do
>>>> get to the point of Just Create One, then heck, you've made the
>>>> lonely decision and committed the sin, you might as well make it
>>>> un-@#$%up-able by some later engineer and enforce it.
>>>
>>> Sorry, I don't understand what you are getting at here. Are you
>>> arguing that there is no difference in coupling between using a
>>> Singletong and a Just Create One?
>>>
>>> Curious, Ilja
>>
>>
>> Coupling, shmupling. No, I'm saying that there is little difference
>> in building an entire design around a JCO by convention vs. building
>> an entire design around a JCO by fiat.
>
> Is "JCO by fiat" the Singleton pattern here? If not, you managed to
> confuse me even more... ;)
>
> BTW, I think that the whole point of JCO is that it's exactly *not*
> the entire design that is build around it. Actually almost all of its
> clients don't know about the number of instances.

I suppose that's true. If JCO means that the JCO instance is passed around
like any other instance, and that the users of that reference don't
particularly regard it as any different than any other instance, with no
assumption as to whether it is only one or one of many, then I suppose
you're right. Tight coupling is avoided, as you suggested.

However, I still just can't get worked up over how hard it is to refactor a
singleton hierarchy into a multi-ton :), should the need arise later. And I
just can't imagine that the cases where it /does/ prove to be
extraordinarily difficult are all that common.

>
>> Like I said before: "eh."
>
> Yes, you keep saying. Doesn't seem to help me understand your point,
> though...

Ok, maybe "bah" then ? :)

--
Framsticks. 3D Artificial Life evolution. You can see the creatures
that evolve and how they interact, hunt, swim, etc. (Unaffiliated with
me). http://www.frams.alife.pl/

From:	Ilja Preuß
Subject:	Re: Singletons
Date:	Thu, 20 Jan 2005 22:38:01 +0100

Thomas G. Marshall wrote:

> I suppose that's true. If JCO means that the JCO instance is passed
> around like any other instance, and that the users of that reference
> don't particularly regard it as any different than any other
> instance, with no assumption as to whether it is only one or one of
> many, then I suppose you're right. Tight coupling is avoided, as you
> suggested.

Exactly! :)

> However, I still just can't get worked up over how hard it is to
> refactor a singleton hierarchy into a multi-ton :), should the need
> arise later.

When the Singleton is referenced from a trillion places in the system, it's
surprisingly much work. And it's quite easy to turn up with such a system
once you introduced the Singleton, in my experience...

> And I just can't imagine that the cases where it /does/
> prove to be extraordinarily difficult are all that common.

Actually it's already more difficult to simply unit test a client of a
Singleton than I'd like it to be. YMMV, of course.

Cheers, Ilja

From:	Calum Grant
Subject:	Re: Singletons
Date:	Sat, 15 Jan 2005 16:15:31 GMT

Ilja Preuß wrote:
> http://butunclebob.com/ArticleS.UncleBob.SingletonVsJustCreateOne
>
> Cheers, Ilja
>
>

Ah, yes. I agree pretty whole-heartedly with that article. Passing the
objects you need around the program is much better design IMHO.
Singletons are a throw-back from procedural programming when global
variables were hard to avoid.

Calum

From:	Ilja Preuß
Subject:	Re: Singletons
Date:	Sun, 16 Jan 2005 00:26:12 +0100

Kent Beck's "Test Driven Development - By Example" also has a chapter on
Design Patterns. My favorite is the entry on Singleton:

"How do you provide global variables in languages without global variables?
Don't. Your programs will thank you for taking the time to think about
design instead."

Cheers, Ilja

From:	iamfractal at hotmail.com
Subject:	Re: Singletons
Date:	20 Jan 2005 00:17:48 -0800

Doc O'Leary wrote:
> In article <41ed86f8@news.totallyobjects.com>,
> "Ilja Preu=DF" wrote:

Snip, snip.

>
> Perhaps. My other related point is that since a class object *is* a
> singleton, I question the need of creating an instance at all when
the
> class itself is just sitting there to use without having to pretend
to
> create a new instance object. Consider, for example, prototype-based
OO
> languages where there is no distinction between classes and
instances.
> In that kind of environment, *any* object in the global scope that
> doesn't serve as a prototype for other objects can be considered a
> singleton. But that is not in any way "special" in a manner that
> suggest it should be avoided.

(I love this thread.)

Just on that point, in languages that distinguish between class and
object, it can be useful to create the single instance rather than
offer class-based access.

For example, in Java (yeah, yeah, I can hear the groans), static
methods can't be overridden by subclasses, so polymorphism gets a
luxury holiday in Guantanamo Bay.

(Though, strictly-speaking, a Singleton's private constructor also
famously makes it unsubclassable in Java, so I personally skip that
bit, which exposes Singletons to neglect from within their package;
more engineering compromise.)
..ed

www.EdmundKirwan.com - Home of The Fractal Class Composition.

From:	Thomas G. Marshall
Subject:	Re: Singletons
Date:	Sat, 22 Jan 2005 14:37:47 GMT

iamfractal@hotmail.com coughed up:
> Doc O'Leary wrote:
>> In article <41ed86f8@news.totallyobjects.com>,
>> "Ilja Preuß" wrote:
>
> Snip, snip.
>
>>
>> Perhaps. My other related point is that since a class object *is* a
>> singleton, I question the need of creating an instance at all when
>> the class itself is just sitting there to use without having to
>> pretend to create a new instance object. Consider, for example,
>> prototype-based OO languages where there is no distinction between
>> classes and instances. In that kind of environment, *any* object in
>> the global scope that doesn't serve as a prototype for other objects
>> can be considered a singleton. But that is not in any way "special"
>> in a manner that suggest it should be avoided.
>
> (I love this thread.)

It has its plusses...

> Just on that point, in languages that distinguish between class and
> object, it can be useful to create the single instance rather than
> offer class-based access.
>
> For example, in Java (yeah, yeah, I can hear the groans),

Whoa, perspective check: the groans I hear about java usually come from OO
purists. They flock to c.o certainly, but they often disagree on so much
regarding other languages that it hardly makes any groans here particularly
poignant.

....[rip]...

--
"So I just, uh... I just cut them up like regular chickens?"
"Sure, just cut them up like regular chickens."

From:	Doc O'Leary
Subject:	Re: Singletons
Date:	Sun, 16 Jan 2005 12:50:45 -0600

In article ,
Calum Grant wrote:

> So my point is this: singletons should be discouraged, even banned.

That is a very short-sighted opinion. Bad coding styles abound, whether
they contain OO singletons/globals or not. What I don't see being
discussed in this thread is how classes themselves, especially in
languages where classes are themselves objects, can be considered
singletons. Are you really calling for a ban on classes? Or possibly
even a ban on namespaces? The deeper question really seems to be that
of resource location, and if you propose to ban global identifiers you
had damn well better propose an alternative that makes more sense for
the developer. Until you do, consider your opinion considered and
rejected.

From:	Ilja Preuß
Subject:	Re: Singletons
Date:	Sun, 16 Jan 2005 23:32:59 +0100

Doc O'Leary wrote:

> Bad coding styles abound,
> whether they contain OO singletons/globals or not.

Well, yes. That's hardly a good argument *for* the use of Singletons,
though.

> What I don't see
> being discussed in this thread is how classes themselves, especially
> in languages where classes are themselves objects, can be considered
> singletons.

I don't think they cause as much trouble as object Singletons, but actually
there are reasons for the existence of principles like "program to an
interface" and patterns like Abstract Factory.

> Are you really calling for a ban on classes? Or possibly
> even a ban on namespaces?

Just because you think that those things have something in common doesn't
mean that from the OPs point of view they cause the same kind of trouble,
does it?

Cheers, Ilja

From:	Doc O'Leary
Subject:	Re: Singletons
Date:	Mon, 17 Jan 2005 11:22:42 -0600

In article <41eaeeeb@news.totallyobjects.com>,
"Ilja Preuß" wrote:

> Doc O'Leary wrote:
>
> > Bad coding styles abound,
> > whether they contain OO singletons/globals or not.
>
> Well, yes. That's hardly a good argument *for* the use of Singletons,
> though.

I was not arguing in favor of singletons, but rather *against* a
campaign to focus on them as a major problem.

> > Are you really calling for a ban on classes? Or possibly
> > even a ban on namespaces?
>
> Just because you think that those things have something in common doesn't
> mean that from the OPs point of view they cause the same kind of trouble,
> does it?

It should. It is inconsistent to consider a singleton bad because it is
"global" yet somehow gloss over other objects that have a global scope.
The only reason a singleton can be considered global is because the
*class* returning it is global. That would be were to address the
issue, and in a more significant way that "just say no".

From:	Ilja Preuß
Subject:	Re: Singletons
Date:	Mon, 17 Jan 2005 22:35:56 +0100

Doc O'Leary wrote:
> It is inconsistent to consider a singleton bad because it
> is "global" yet somehow gloss over other objects that have a global
> scope.

Class instances are used for other things and in other ways than the other
"objects" you mentioned, and therefore globality does cause different (in my
opinion more) problems. No inconsistency or glossing over involved, as far
as I can see.

Take care, Ilja

From:	Doc O'Leary
Subject:	Re: Singletons
Date:	Tue, 18 Jan 2005 12:39:31 -0600

In article <41ec330b$1@news.totallyobjects.com>,
"Ilja Preuß" wrote:

> Doc O'Leary wrote:
> > It is inconsistent to consider a singleton bad because it
> > is "global" yet somehow gloss over other objects that have a global
> > scope.
>
> Class instances are used for other things and in other ways than the other
> "objects" you mentioned, and therefore globality does cause different (in my
> opinion more) problems. No inconsistency or glossing over involved, as far
> as I can see.

Just because you choose to *use* them differently doesn't *make* them
different. A class *is* global, whether it makes one instance, two
instances (e.g., a boolean), or any number of otherwise uniqued
instances (e.g., immutable strings). Worrying about the details of what
sort of instance gets returned is ludicrous. All I'm asking is that you
argue against the right thing, and it seems to me that singletons are
*not* the root of whatever issue is being raised.

From:	Thomas G. Marshall
Subject:	Re: Singletons
Date:	Fri, 21 Jan 2005 00:46:28 GMT

Doc O'Leary coughed up:
> In article <41ec330b$1@news.totallyobjects.com>,
> "Ilja Preuß" wrote:
>
>> Doc O'Leary wrote:
>>> It is inconsistent to consider a singleton bad because it
>>> is "global" yet somehow gloss over other objects that have a global
>>> scope.
>>
>> Class instances are used for other things and in other ways than the
>> other "objects" you mentioned, and therefore globality does cause
>> different (in my opinion more) problems. No inconsistency or
>> glossing over involved, as far as I can see.
>
> Just because you choose to *use* them differently doesn't *make* them
> different. A class *is* global, whether it makes one instance, two
> instances (e.g., a boolean), or any number of otherwise uniqued
> instances (e.g., immutable strings). Worrying about the details of
> what sort of instance gets returned is ludicrous. All I'm asking is
> that you argue against the right thing, and it seems to me that
> singletons are *not* the root of whatever issue is being raised.

Well, as you can tell from my other posts here, I too think this concern
over singletons is a little silly.

But in all honesty, I don't think your logic tracks here precisely. A
singleton is something created by the engineer with a specific purpose: to
be only one. A class, while there is only one, has a much greater use in
stamping out multiple instances. To say that all class access is fraught
with the precise same implications as singleton use is not entirely true.
That is, you can get rid of all singletons---you can't get rid of all
classes----/those/ are irreplaceable (except for oddball OOPL's).

But then (again), I don't think /any/ of this is an issue.

--
"His name was Robert Paulson. His name was Robert Paulson. His name was
Robert Paulson..."

From:	Doc O'Leary
Subject:	Re: Singletons
Date:	Fri, 21 Jan 2005 15:05:36 -0600

In article ,
"Thomas G. Marshall"
wrote:

> But in all honesty, I don't think your logic tracks here precisely. A
> singleton is something created by the engineer with a specific purpose: to
> be only one. A class, while there is only one, has a much greater use in
> stamping out multiple instances.

But that's why my logic tracks so very well! Just because a class can
conceptually produce many instances is irrelevant in the case of a
singleton. For that, you have precisely one class object and one
instance object, leaving me to wonder why you need the instance object
at all.

> But then (again), I don't think /any/ of this is an issue.

Likewise. I don't have have any real problem with a class/instance
"duality" singleton because there are, as others have noted, some OO
languages that treat class objects differently. For my own uses,
though, I prefer to use the class object as the singleton and forego all
the unnecessary busywork of maintaining a single instance as well.

From:	Thomas Gagne
Subject:	Re: Singletons
Date:	Sat, 22 Jan 2005 10:43:06 -0500

Doc O'Leary wrote:

>
>
> Likewise. I don't have have any real problem with a class/instance
> "duality" singleton because there are, as others have noted, some OO
> languages that treat class objects differently. For my own uses,
> though, I prefer to use the class object as the singleton and forego all
> the unnecessary busywork of maintaining a single instance as well.

That seems to make sense, and would be very easy to do in Smalltalk, but I
haven't noticed it as an idiom and didn't find it in the index of Beck's "Best
Practice Patterns."

From:	Ilja Preuß
Subject:	Re: Singletons
Date:	Sat, 22 Jan 2005 18:09:34 +0100

Thomas Gagne wrote:
> That seems to make sense, and would be very easy to do in Smalltalk,
> but I haven't noticed it as an idiom and didn't find it in the index
> of Beck's "Best Practice Patterns."

Very good book!

From his "Test Driven Development - by Example" book, it becomes quite clear
that Beck doesn't like Singletons.

Cheers, Ilja

From:	Thomas Gagne
Subject:	Re: Singletons
Date:	Sun, 23 Jan 2005 10:49:29 -0500

Ilja Preuß wrote:
> Thomas Gagne wrote:
>

> From his "Test Driven Development - by Example" book, it becomes quite clear
> that Beck doesn't like Singletons.

Is that because he doesn't like singletons or because it's easier to use class
variables?

From:	Ilja Preuß
Subject:	Re: Singletons
Date:	Sun, 23 Jan 2005 17:22:27 +0100

Thomas Gagne wrote:
> Ilja Preuß wrote:
>> Thomas Gagne wrote:
>>
>
>> From his "Test Driven Development - by Example" book, it becomes
>> quite clear that Beck doesn't like Singletons.
>
> Is that because he doesn't like singletons or because it's easier to
> use class variables?

The book has a chapter on a number of design patterns, with approximately
one page per pattern.

Singleton is the only exception. Kent Beck writes about it: "How do you
provide global variables in languages without global variables? Don't. Your
programs will thank you for taking the time to think about design instead."

Cheers, Ilja

From:	Michael Feathers
Subject:	Re: Singletons
Date:	Sun, 23 Jan 2005 16:28:22 GMT

"Ilja Preuß" wrote in message
news:41f3d2a0@news.totallyobjects.com...
> Thomas Gagne wrote:
> > Ilja Preuß wrote:
> >> Thomas Gagne wrote:
> >>
> >
> >> From his "Test Driven Development - by Example" book, it becomes
> >> quite clear that Beck doesn't like Singletons.
> >
> > Is that because he doesn't like singletons or because it's easier to
> > use class variables?
>
> The book has a chapter on a number of design patterns, with approximately
> one page per pattern.
>
> Singleton is the only exception. Kent Beck writes about it: "How do you
> provide global variables in languages without global variables? Don't.
Your
> programs will thank you for taking the time to think about design
instead."

It's a pretty important point. When you use globals often you miss
opportunities to really think about layering.

--
Michael Feathers
author, Working Effectively with Legacy Code (Prentice Hall 2005)
www.objectmentor.com

From:	Roger L. Cauvin
Subject:	Re: Singletons
Date:	Sun, 23 Jan 2005 11:00:44 -0600

"Thomas Gagne" wrote in message
news:OIydnSZDwsIZWm7cRVn-gw@wideopenwest.com...
> Ilja Preuß wrote:
>> Thomas Gagne wrote:
>>
>
>> From his "Test Driven Development - by Example" book, it
>> becomes quite clear that Beck doesn't like Singletons.
>
> Is that because he doesn't like singletons or because it's easier
> to use class variables?

Thomas, I think you need to reflect on your tendency to rationalize away
quotes from authoritative sources. As another reply in this message
implies, Beck suggests avoiding singletons, class variables, and anything
resembling a global variable.

In a singleton thread a couple of years ago, you attempted to rationalize
away the following quote from Ralph Johnson, one of the original GoF who
wrote the DESIGN PATTERNS book:

"Singleton is a weak pattern, and whenever people start with Singleton, it
is a sign of a bad design."

At the time, you chose to escape as follows:

"Dr. Johnson was speaking in the context of designing a GUI. I'm unsure if a
single sentence applies to everything."

Recognize your own rationalization "pattern"?

--
Roger L. Cauvin
nospam_roger@cauvin.org (omit the "nospam_" part)
Cauvin, Inc.
http://www.cauvin-inc.com

From:	Thomas G. Marshall
Subject:	Re: Singletons
Date:	Mon, 24 Jan 2005 04:27:38 GMT

Roger L. Cauvin coughed up:
> "Thomas Gagne" wrote in message
> news:OIydnSZDwsIZWm7cRVn-gw@wideopenwest.com...
>> Ilja Preuß wrote:
>>> Thomas Gagne wrote:
>>>
>>
>>> From his "Test Driven Development - by Example" book, it
>>> becomes quite clear that Beck doesn't like Singletons.
>>
>> Is that because he doesn't like singletons or because it's easier
>> to use class variables?
>
> Thomas, I think you need to reflect on your tendency to rationalize
> away quotes from authoritative sources. As another reply in this
> message implies, Beck suggests avoiding singletons, class variables,
> and anything resembling a global variable.
>
> In a singleton thread a couple of years ago, you attempted to
> rationalize away the following quote from Ralph Johnson, one of the
> original GoF who wrote the DESIGN PATTERNS book:
>
> "Singleton is a weak pattern, and whenever people start with
> Singleton, it is a sign of a bad design."
>
> At the time, you chose to escape as follows:
>
> "Dr. Johnson was speaking in the context of designing a GUI. I'm
> unsure if a single sentence applies to everything."
>
> Recognize your own rationalization "pattern"?

I'm not sure that's a real problem. It doesn't sound like rationalization
to me; more like simple qualification.

--
Whyowhydidn'tsunmakejavarequireanuppercaselettertostartclassnames....

From:	Michael Feathers
Subject:	Re: Singletons
Date:	Sun, 23 Jan 2005 21:47:47 GMT

"Roger L. Cauvin" wrote in message
news:35i3hqF4gfdmbU1@individual.net...
> "Thomas Gagne" wrote in message
> news:OIydnSZDwsIZWm7cRVn-gw@wideopenwest.com...
> > Ilja Preuß wrote:
> >> Thomas Gagne wrote:
> >>
> >
> >> From his "Test Driven Development - by Example" book, it
> >> becomes quite clear that Beck doesn't like Singletons.
> >
> > Is that because he doesn't like singletons or because it's easier
> > to use class variables?
>
> Thomas, I think you need to reflect on your tendency to rationalize away
> quotes from authoritative sources. As another reply in this message
> implies, Beck suggests avoiding singletons, class variables, and anything
> resembling a global variable.
>
> In a singleton thread a couple of years ago, you attempted to rationalize
> away the following quote from Ralph Johnson, one of the original GoF who
> wrote the DESIGN PATTERNS book:
>
> "Singleton is a weak pattern, and whenever people start with Singleton, it
> is a sign of a bad design."
>
> At the time, you chose to escape as follows:
>
> "Dr. Johnson was speaking in the context of designing a GUI. I'm unsure if
a
> single sentence applies to everything."
>
> Recognize your own rationalization "pattern"?

I don't really mind that. Things are true or false independently of whether
someone famous said it.

--
Michael Feathers
author, Working Effectively with Legacy Code (Prentice Hall 2005)
www.objectmentor.com

From:	Thomas Gagne
Subject:	Re: Singletons
Date:	Sun, 23 Jan 2005 17:26:16 -0500

Roger L. Cauvin wrote:

>
> Thomas, I think you need to reflect on your tendency to rationalize away
> quotes from authoritative sources. As another reply in this message
> implies, Beck suggests avoiding singletons, class variables, and anything
> resembling a global variable.

I was actually curious and not trying to trivialize the advice. If he ignored
a pattern I thought he may have provided an alternative, or suggested it
wasn't necessary given another idiom, or that there may be some other
explanation. The quote given was a pleasant platitude but doesn't provide any
rationalization of its own (I don't have the book so can't look up its context).
>
> In a singleton thread a couple of years ago, you attempted to rationalize
> away the following quote from Ralph Johnson, one of the original GoF who
> wrote the DESIGN PATTERNS book:
>
> "Singleton is a weak pattern, and whenever people start with Singleton, it
> is a sign of a bad design."
>
> At the time, you chose to escape as follows:
>
> "Dr. Johnson was speaking in the context of designing a GUI. I'm unsure if a
> single sentence applies to everything."
>
> Recognize your own rationalization "pattern"?
>

Go figure. I could write my own patterns book!

Regardless of the authority (though I admit a tendency to give their words
more weight than others') I think it's inadequate to simply say something is
fundamentally bad without some exposition on why.

I've used singletons twice in five years. The simplest to explain is a class
that wraps standard-io for Smalltalk. To access the standard IO streams,
which could be accessed nearly anywhere by anyone, a simple "EfStdIO default
stdout" would return the standard output stream. There are advantages to
having it be an instance as it relates to construction and destruction of the
streams than implementing everything as a class method or having multiples of
them.

So I'm still wondering about the rules, which ones I may have violated, and
how much time I may have wasted looking for something other than a singleton
had I read advice warning they were evil, when perhaps they were the simlest
correct thing.

I'll have to reread that thread to rediscover if Dr. Johnson meant that a
Singleton was an indicator of bad design anywhere it might be used.

As was noted earlier in this thread, there are lots of globals in language
environments (classes, namespaces), in logical address spaces (any named
address, function, etc.), and on computers. There must be a better rule than
"all the globals that exist are all that ever need exist and any you feel
necessary are not." That smells a lot like language design philosophies that
prohibit extending classes or overriding methods. Few designs are so
perfectly conceived and implemented.

From:	Thomas G. Marshall
Subject:	Re: Singletons
Date:	Sat, 22 Jan 2005 14:55:16 GMT

Doc O'Leary coughed up:
> In article ,
> "Thomas G. Marshall"
> wrote:
>
>> But in all honesty, I don't think your logic tracks here precisely.
>> A singleton is something created by the engineer with a specific
>> purpose: to be only one. A class, while there is only one, has a
>> much greater use in stamping out multiple instances.
>
> But that's why my logic tracks so very well! Just because a class can
> conceptually produce many instances is irrelevant in the case of a
> singleton. For that, you have precisely one class object and one
> instance object, leaving me to wonder why you need the instance object
> at all.
>
>> But then (again), I don't think /any/ of this is an issue.
>
> Likewise. I don't have have any real problem with a class/instance
> "duality" singleton because there are, as others have noted, some OO
> languages that treat class objects differently. For my own uses,
> though, I prefer to use the class object as the singleton and forego
> all the unnecessary busywork of maintaining a single instance as well.

I've seen that a lot. I (too) don't see a hell of a lot of difference
between

Singleton.instance.hoohah();

and

Singleton.hoohah();

With the exception that the former Singleton.instance can be passed about
like a normal instance where the latter cannot in most OOPL's I think. JCO
arguments aside.

--
"So I just, uh... I just cut them up like regular chickens?"
"Sure, just cut them up like regular chickens."

From:	Ilja Preuß
Subject:	Re: Singletons
Date:	Sat, 22 Jan 2005 17:58:57 +0100

Thomas G. Marshall wrote:
> I (too) don't see a hell of a lot of difference
> between
>
> Singleton.instance.hoohah();
>
> and
>
> Singleton.hoohah();
>
> With the exception that the former Singleton.instance can be passed
> about like a normal instance where the latter cannot in most OOPL's I
> think.

And Singleton.instance could actually give us an instance of a *subclass* of
Singleton, without the clients noticing. Often that is the only way to make
the clients of a Singleton testable.

Cheers, Ilja

From:	Doc O'Leary
Subject:	Re: Singletons
Date:	Sat, 22 Jan 2005 17:38:04 -0600

In article <41f289ad$1@news.totallyobjects.com>,
"Ilja Preuß" wrote:

> Thomas G. Marshall wrote:
> >
> > With the exception that the former Singleton.instance can be passed
> > about like a normal instance where the latter cannot in most OOPL's I
> > think.

Sure, but the shortcomings of an implementation language should not
handicap a good design. Abstractly a singleton is "just one", and if
you happen to be using an environment that can appropriately represent
that with one (class) object, it makes no sense to complicate the
implementation by using an unnecessary instance.

> And Singleton.instance could actually give us an instance of a *subclass* of
> Singleton, without the clients noticing.

The same can be accomplished in OO environments that allow classes to
pose as other classes. It also means it can also be done without
changing the first class at all, whereas your way would force the it to
know about the subclass in order to return its instance. Again, when
possible, it seems just using the class as the singleton directly is a
better approach.

From:	Ilja Preuß
Subject:	Re: Singletons
Date:	Sun, 23 Jan 2005 01:41:36 +0100

Doc O'Leary wrote:

>> And Singleton.instance could actually give us an instance of a
>> *subclass* of Singleton, without the clients noticing.
>
> The same can be accomplished in OO environments that allow classes to
> pose as other classes.

As I said, I've never worked in such a language, so I can't comment.

Cheers, Ilja

From:	Ilja Preuß
Subject:	Re: Singletons
Date:	Tue, 18 Jan 2005 22:46:03 +0100

Doc O'Leary wrote:
> In article <41ec330b$1@news.totallyobjects.com>,
> "Ilja Preuß" wrote:
>
>> Doc O'Leary wrote:
>>> It is inconsistent to consider a singleton bad because it
>>> is "global" yet somehow gloss over other objects that have a global
>>> scope.
>>
>> Class instances are used for other things and in other ways than the
>> other "objects" you mentioned, and therefore globality does cause
>> different (in my opinion more) problems. No inconsistency or
>> glossing over involved, as far as I can see.
>
> Just because you choose to *use* them differently doesn't *make* them
> different.

But how I use something changes what properties I want it to have, and what
properties present a problem for me.

Both my old CD-ROM drive and my hifi system are quite loud. I think it's
much more of a problem for the former.

> A class *is* global, whether it makes one instance, two
> instances (e.g., a boolean), or any number of otherwise uniqued
> instances (e.g., immutable strings).

Yes, it is. I don't see your point though. Perhaps you'd like to explain how
that property of a class leads to the same problems the globality of the
access point to the single instance of a Singleton does.

> Worrying about the details of
> what sort of instance gets returned is ludicrous.

I can't identify with that statement, mostly because I don't understand it.
But I have the gut feel that you misunderstand what I dislike about the
Singleton pattern.

> All I'm asking is
> that you argue against the right thing, and it seems to me that
> singletons are *not* the root of whatever issue is being raised.

The *root* of the issue - to me - is an overly coupled design that makes a
system hard to change. The singleton pattern is just a very specific
incarnation of that problem, one that is prominent enough to be discussed
explicitely, in my opinion.

Take care, Ilja

From:	Doc O'Leary
Subject:	Re: Singletons
Date:	Wed, 19 Jan 2005 15:27:45 -0600

In article <41ed86f8@news.totallyobjects.com>,
"Ilja Preuß" wrote:

> Doc O'Leary wrote:
> >
> > Just because you choose to *use* them differently doesn't *make* them
> > different.
>
> But how I use something changes what properties I want it to have, and what
> properties present a problem for me.

No it doesn't! We're talking about a singleton here. If you have
reason to use it differently, it should not be the same object. That
would mean the real issue is you using singletons where they are not the
correct solution.

> Both my old CD-ROM drive and my hifi system are quite loud. I think it's
> much more of a problem for the former.

I don't understand this example.

> > A class *is* global, whether it makes one instance, two
> > instances (e.g., a boolean), or any number of otherwise uniqued
> > instances (e.g., immutable strings).
>
> Yes, it is. I don't see your point though. Perhaps you'd like to explain how
> that property of a class leads to the same problems the globality of the
> access point to the single instance of a Singleton does.

I can't explain because I don't see a singleton as a problem. You do,
so you it is on your shoulder to explain why access to a single instance
is bad, but access to the *class* of that instance, something that is
usually necessary to *get* that singleton, is not bad.

> > Worrying about the details of
> > what sort of instance gets returned is ludicrous.
>
> I can't identify with that statement, mostly because I don't understand it.
> But I have the gut feel that you misunderstand what I dislike about the
> Singleton pattern.

Perhaps. My other related point is that since a class object *is* a
singleton, I question the need of creating an instance at all when the
class itself is just sitting there to use without having to pretend to
create a new instance object. Consider, for example, prototype-based OO
languages where there is no distinction between classes and instances.
In that kind of environment, *any* object in the global scope that
doesn't serve as a prototype for other objects can be considered a
singleton. But that is not in any way "special" in a manner that
suggest it should be avoided.

From:	Ilja Preuß
Subject:	Re: Singletons
Date:	Thu, 20 Jan 2005 14:32:11 +0100

Doc O'Leary wrote:
> In article <41ed86f8@news.totallyobjects.com>,
> "Ilja Preuß" wrote:
>
>> Doc O'Leary wrote:
>>>
>>> Just because you choose to *use* them differently doesn't *make*
>>> them different.
>>
>> But how I use something changes what properties I want it to have,
>> and what properties present a problem for me.
>
> No it doesn't! We're talking about a singleton here.

Sorry for not being clear - I'm talking about the fact that just because
being global is a problem for a class instance, it doesn't mean that being
global is a similarly big problem for everything else, too.

>> Both my old CD-ROM drive and my hifi system are quite loud. I think
>> it's much more of a problem for the former.
>
> I don't understand this example.

Reconsider in the light of the above clarification.

>>> A class *is* global, whether it makes one instance, two
>>> instances (e.g., a boolean), or any number of otherwise uniqued
>>> instances (e.g., immutable strings).
>>
>> Yes, it is. I don't see your point though. Perhaps you'd like to
>> explain how that property of a class leads to the same problems the
>> globality of the access point to the single instance of a Singleton
>> does.
>
> I can't explain because I don't see a singleton as a problem. You do,
> so you it is on your shoulder to explain why access to a single
> instance
> is bad, but access to the *class* of that instance, something that is
> usually necessary to *get* that singleton, is not bad.

When using JCO instead of Singleton, a client doesn't access anything
global, but is given the instance to work with from the outside. That
reduces coupling and thereby increases flexibility.

> Perhaps. My other related point is that since a class object *is* a
> singleton, I question the need of creating an instance at all when the
> class itself is just sitting there to use without having to pretend to
> create a new instance object.

Well, sort of. In some languages, like Java, using class methods provides
even less flexibility, because they aren't polymorphic.

Directly using class methods, though, certainly presents similar problems to
those of the Singleton pattern, namely a coupling of the client to that
particular class.

> Consider, for example, prototype-based
> OO languages where there is no distinction between classes and
> instances.
> In that kind of environment, *any* object in the global scope that
> doesn't serve as a prototype for other objects can be considered a
> singleton. But that is not in any way "special" in a manner that
> suggest it should be avoided.

Sorry, I don't know enough about prototype based languages to comment.

Cheers, Ilja

From:	Andrew McDonagh
Subject:	Re: Singletons
Date:	Tue, 18 Jan 2005 19:43:04 +0000

Doc O'Leary wrote:
> In article <41ec330b$1@news.totallyobjects.com>,
> "Ilja Preuß" wrote:
>
>
>>Doc O'Leary wrote:
>>
>>>It is inconsistent to consider a singleton bad because it
>>>is "global" yet somehow gloss over other objects that have a global
>>>scope.
>>
>>Class instances are used for other things and in other ways than the other
>>"objects" you mentioned, and therefore globality does cause different (in my
>>opinion more) problems. No inconsistency or glossing over involved, as far
>>as I can see.
>
>
> Just because you choose to *use* them differently doesn't *make* them
> different. A class *is* global, whether it makes one instance, two
> instances (e.g., a boolean), or any number of otherwise uniqued
> instances (e.g., immutable strings). Worrying about the details of what
> sort of instance gets returned is ludicrous. All I'm asking is that you
> argue against the right thing, and it seems to me that singletons are
> *not* the root of whatever issue is being raised.

You are right...

Right in that Singletons are not bad - and most replies have stated that
we don't consider Singletons to be wrong/bad/evil/etc.

However, most replies have shown the misuse of Singletons, and its this
misuse which is bad/wrong/evil/etc.

Personally, if a design merits a Singleton then I will use a Singleton,
but I will want to be completely sure that it is ABSOLUTELY VITAL that
only a single instance of the class can exist. I certainly won't use a
singleton because we ONLY need a single instance today, nor will I use a
Singleton because its static access method makes it easier to get to it.

From:	Andrew McDonagh
Subject:	Re: Singletons
Date:	Sat, 15 Jan 2005 19:34:56 +0000

Calum Grant wrote:
> I quite often see singletons in code, and I think they are overused.
>
> Singletons have the disadvantage that everyone in the whole world has
> access to that object, which is as bad as a global function. Anyone can
> call a singleton and create havoc on it.
>
> Singletons also introduce hard-coded dependencies that are difficult to
> break if one wishes to change the environment or reuse some of the
> classes in a different project.
>
> Just because you call something a "singleton" instead of a global
> variable does not make it a good pattern.
>
> Singletons are lazy design, since "global scope" is too broad a scope
> for most objects.
>
> So my point is this: singletons should be discouraged, even banned.
>
> Calum

Whilst I understand the need for the pattern and have used it once or
twice myself, I fully agree with you. The JustCreateOne alternative can
have a much better effect upon the code base and its design.

Just this week, I've been refactoring a huge section of code to rid it
of a singleton, because we now need two instances of the class in the
system. The fact that we never did actually need to prevent more than
one instance before, is a classic example of the developers choosing a
singleton, not because its appropriate, but because it was easier to
have a global object and its static getInstance() method.

The situation was actually worse, in that the guys needing to have a
second instance of the object around, but not being able to see an easy
way of refactoring the singleton away. So they made a public
setInstance() method to accompany its getInstance(). :-)

From:	smilemac
Subject:	Re: Singletons
Date:	Sun, 16 Jan 2005 18:50:38 +0800

"Andrew McDonagh" wrote in message
news:csbr93$2vr$1@news.freedom2surf.net...
> Calum Grant wrote:
> > I quite often see singletons in code, and I think they are overused.
> >
> > Singletons have the disadvantage that everyone in the whole world has
> > access to that object, which is as bad as a global function. Anyone can
> > call a singleton and create havoc on it.
> >
> > Singletons also introduce hard-coded dependencies that are difficult to
> > break if one wishes to change the environment or reuse some of the
> > classes in a different project.
> >
> > Just because you call something a "singleton" instead of a global
> > variable does not make it a good pattern.
> >
> > Singletons are lazy design, since "global scope" is too broad a scope
> > for most objects.
> >
> > So my point is this: singletons should be discouraged, even banned.
> >
> > Calum
>
> Whilst I understand the need for the pattern and have used it once or
> twice myself, I fully agree with you. The JustCreateOne alternative can
> have a much better effect upon the code base and its design.
>
> Just this week, I've been refactoring a huge section of code to rid it
> of a singleton, because we now need two instances of the class in the
> system. The fact that we never did actually need to prevent more than
> one instance before, is a classic example of the developers choosing a
> singleton, not because its appropriate, but because it was easier to
> have a global object and its static getInstance() method.
>
> The situation was actually worse, in that the guys needing to have a
> second instance of the object around, but not being able to see an easy
> way of refactoring the singleton away. So they made a public
> setInstance() method to accompany its getInstance(). :-)

Could you give more details about why it's not easy to refactor? Why you
cannot just remove the "modifiers" for singleton, such as "static",
"private/protected" and etc. ? What purpose is the "setInstance", to reset
the instance as null? If so, I think your first version design should have
some problems.

smilemac

From:	Andrew McDonagh
Subject:	Re: Singletons
Date:	Sun, 16 Jan 2005 13:21:47 +0000

smilemac wrote:
> "Andrew McDonagh" wrote in message

snipped.

>>
>>Whilst I understand the need for the pattern and have used it once or
>>twice myself, I fully agree with you. The JustCreateOne alternative can
>>have a much better effect upon the code base and its design.
>>
>>Just this week, I've been refactoring a huge section of code to rid it
>>of a singleton, because we now need two instances of the class in the
>>system. The fact that we never did actually need to prevent more than
>>one instance before, is a classic example of the developers choosing a
>>singleton, not because its appropriate, but because it was easier to
>>have a global object and its static getInstance() method.
>>
>>The situation was actually worse, in that the guys needing to have a
>>second instance of the object around, but not being able to see an easy
>>way of refactoring the singleton away. So they made a public
>>setInstance() method to accompany its getInstance(). :-)
>
>
> Could you give more details about why it's not easy to refactor?

It was so much as hard to refactor, but time consuming just because of
the number of classes using the singleton. The hard-ish part was looking
at the code and deciding how these classes could get access to 'an'
instance of the class, rather than 'theSingleton' instance.

Some questions we asked ourselves were...
#Should the instance be passed directly to them?
#Should they get() it from another object they already had access too?
#Should we use a Register type pattern to still have global access, not
to the singleton object, but to a Registry of these objects, keyed off
some value?
#etc.

In the end we decided with the current design the easiest, quickest,
smartest approach was for the objects to get() 'an' instance of the
singleton from an associated object.

>Why you
> cannot just remove the "modifiers" for singleton, such as "static",
> "private/protected" and etc. ?

The code using the public static access method : getInstance() is spread
around the code base. Depending upon the context the client classes are
in, they all needed to perform actions upon the same instance of the
object which currently is a singleton. However, there was never any
need for a singleton. The same set of client classes, with a different
context, are need to perform the same work but on the different instance
of the class which is currently a singleton.

There's now only a 3 classes( and their tests) still to change. Once
they have been refactored, we will be free to simply remove the static
getInstance() method from the singleton class, and simply 'new' an
instance whenever and wherever we need too.

What purpose is the "setInstance", to reset
> the instance as null? If so, I think your first version design should have
> some problems.
>

It simply changed the object that was returned via the getInstance()
method - basically to 'allow' a second instance of the singleton class
to exist.

This was a band aid 'fix' to enable the new requirement to be
implemented, without having to spend time refactoring the original
design which inappropriately used a singleton.

The original design inappropriately used a singleton, because it was the
simplest and quickest thing to do. We initially saved some development
time. Then as soon as we had a new requirement that needed another
instance of the same class, we 'fixed' the situation with a band aid.
However, this just meant that the cost of refactoring away the
singleton, was now increased further, as there was now more code using it.

Its my estimation that the time its taken me and my pair to refactor
away the singleton, is at least twice that what it would have been to
not use it in the first place, regardless of the extra code generated
for the second requirement, that also needed to be refactored.

All of this, because the Singleton was used, not for its purpose of
controlling instances of the class, but for its global access point
method: getInstance().

From:	smilemac
Subject:	Re: Singletons
Date:	Sun, 16 Jan 2005 00:41:12 +0800

"Calum Grant" wrote in message
news:juaGd.83$VQ5.73@newsfe1-gui.ntli.net...
> I quite often see singletons in code, and I think they are overused.
>
> Singletons have the disadvantage that everyone in the whole world has
> access to that object, which is as bad as a global function. Anyone can
> call a singleton and create havoc on it.
>
> Singletons also introduce hard-coded dependencies that are difficult to
> break if one wishes to change the environment or reuse some of the
> classes in a different project.
>
> Just because you call something a "singleton" instead of a global
> variable does not make it a good pattern.
>
> Singletons are lazy design, since "global scope" is too broad a scope
> for most objects.
>
> So my point is this: singletons should be discouraged, even banned.

There are many things be singleton, such as log, your application instance,
your frame window and etc. If you were worrying about the visible scope. You
could use a mediator or proxy to keep it in house. Dose it help?

smilemac

From:	Calum Grant
Subject:	Re: Singletons
Date:	Sat, 15 Jan 2005 17:47:53 GMT

smilemac wrote:
> "Calum Grant" wrote in message
> n